This invention relates in general to ultrasonic measurement systems for ascertaining density or other characteristics of a fluid. More particularly, it relates to determining density of fluids flowing in pipes, including fluids like liquids, gases, and some two-phase combinations or mixtures.
It has been recognized in the ultrasonic literature since the 1960s, and perhaps earlier, that by measuring the acoustic impedance and sound speed of a fluid, one can determine the fluid density, dividing the impedance by the sound speed. In the prior art it is also known that impedance can be obtained from reports of the reflection coefficient at a fluid/solid reflecting interface. The 1982 U.S. Pat. No. 4,320,659 for Ultrasonic System for Measuring Fluid Impedance or Liquid Level of inventors Lynnworth, Seger and Bradshaw, teaches, in connection with FIGS. 4-9 thereof, that the reflecting surface should be substantially flush with the interior surface of the vessel or conduit in which the reflection coefficient is measured. The clamp-on systems of those figures used the existing internal wall of the vessel as the surface at which R is sensed. This sensor was minimally intrusive, and thereby minimally disturbed the process operations or the flow, and introduced little pressure drop. Generally, since a protrusion could be a site for buildup, an intrusive reflector is viewed as undesirable. This idea of flush mounting a reflecting surface is still prevalent, and is found in several recent technical papers on the subject, examples being Van Deventer and Delsing, 1997, An Ultrasonic Density Probe, pp. 871-875 in Proc. 1997 IEEE Ultrasonics Symposium, and the peer-reviewed paper by Adamowski, Buiochi and Sigelmann, Ultrasonic Measurement of Density of Liquids Flowing in Tubes, IEEE Trans UFFC 45 (1), pp. 48-56 (January 1998). These two papers may be taken to represent the current teaching in this field regarding reflective determinations of fluid density. They also teach that, to achieve sensitivity to fluid density, the characteristic acoustic impedance (Z.sub.0) of the reflector should be comparable to that of the fluid. The Z.sub.0 of many liquids is on the order of one to two mks rayls, water having a value of 1.5 in these units. If one scans a handbook of physics table of characteristic acoustic impedances of solids, one finds Z.sub.0 of about 2 to 3 for plastics, and about 17 to 50 for commonly-used metals. This leads one to select a plastic as the reflector. If a metal were to be used, the reflection coefficient R at a metal/fluid interface is typically so close to unity, that the small changes in fluid density that are of interest would be very difficult to detect, let alone measure with high accuracy. This would lead one to conclude that using a metal reflector would not be of much practical use in a reflectometer designed for sensing fluid density. This would appear particularly true for a metal such as stainless steel (SS), having Z.sub.0 about 45 mks rayls, or nickel, with Z.sub.0 of 54 mks rayls. In the 1982 '659 patent mentioned above, a high-Z.sub.0 sensor is shown.
Nonetheless, there remains a need for sensing or determining fluid properties, such as density, or properties such as mass flow rate, that depend on density.